The head of the Senate Judiciary Committee hopes to unmask the identity of the Republican and Democratic political operatives who financed the opposition research of Donald Trump that ended up in a salacious dossier compiled by a former British spy.

In a letter sent out Monday, Iowa Sen. Chuck Grassley asked Glenn Simpson, the head of Fusion GPS, to identify the clients who hired him to dig up dirt on Trump during the campaign.

Grassley also wants Simpson to turn over all contracts related to the research project, including his agreements with Christopher Steele, the former MI6 agent who authored the dossier.

The identities of Fusion’s clients — one an anti-Trump Republican and the other a pro-Hillary Clinton Democrat — have remained a mystery since the dossier was published by BuzzFeed News on Jan. 10. But Grassley says that the backers are important because the politically-motivated research project ended up in the hands of the FBI.

Glenn Simpson, 1996 C-SPAN interview.

“When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics,” Grassley wrote in his letter to Simpson, a former reporter for the Wall Street Journal.

The Republican backer hired Fusion in Sept. 2015 to conduct research on Trump. But as the real estate mogul ascended to the GOP nomination, the financier gave up on the research. Fusion then found a Democratic donor to take over. That’s when Fusion hired Steele to dig into Trump’s activities in Russia.

Steele, who runs Orbis Business Intelligence, produced a series of memos dated from between June 20 and Dec. 13. Using sources based in Russia and the U.S., the former spy reported that the Russian government was colluding with the Trump campaign to influence the election.

While none of the most salacious parts of the dossier have been verified, Democrats in Congress have cited it as fact, most recently in a House Intelligence Committee hearing held last Monday.

One of the alleged sources of the most shocking parts of the dossier is said to be a Belarusian businessman who has been accused of exaggerating his business relationships, including with Trump. The businessman, Sergei Millian, is identified as Source D and Source E in the dossier.

Grassley seeks answers to his letter by April 7. In it, he asks Fusion for details about the clients, when they hired his firm, how much they paid for the work, and how much they paid Steele.

Grassley also wants to know whether Fusion met with the FBI and how the firm distributed the dossier.

The Washington, D.C.-based firm reportedly shared the dossier with a handful of Beltway reporters prior to the election. Mother Jones reporter David Corn did some reporting on the dossier days before the election. He also interviewed Steele. BBC reporter Paul Wood was also given access to the dossier, he reported in January.

It also emerged earlier this month that the FBI struck an informal deal with Steele in October to continue his research on Trump. Steele was never paid, though it is unclear why. Grassley has asked the FBI for details of that arrangement. It was also reported this month that Steele briefed FBI agents on his findings on July 5, just weeks after he compiled the first memo of the dossier.

Content created by The Daily Caller News Foundation is available without charge to any eligible news publisher that can provide a large audience. For licensing opportunities of our original content, please contact [email protected].